The present invention relates generally to methods for reducing strain on optical fiber Bragg gratings, and more particularly, to methods for reducing the strain imposed on the fiber grating as the ambient temperature fluctuates.
A conventional Bragg grating comprises an optical fiber in which the index of refraction undergoes periodic perturbations along its length. The perturbations may be equally spaced, as in the case of an unchirped grating, or may be unequally spaced, as in the case of a chirped grating. The fiber grating reflects light over a given waveband centered around a wavelength equal to twice the spacing between successive perturbations. The remaining wavelengths pass essentially unimpeded. Such fiber Bragg gratings are typically employed in a variety of applications including filtering, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for fiber dispersion.
Fiber gratings are typically mounted on a substrate or wound around a retaining element, which are in turn secured in a housing. Since the fiber grating and the substrate or retaining element generally have different thermal coefficients of expansion, the fiber grating will typically be under tension or stress as the ambient temperature fluctuates. To prolong the expected lifetime of the fiber grating, however, the grating ideally should be packaged in such a way that it is under minimal stress.
Packages for fiber grating often address temperature fluctuations because both the refractive index of the grating and the distance between successive perturbations are temperature dependent. As a result, the reflected waveband is also temperature dependent. In many cases, however, it is desirable to provide a stabilized reflection band that is temperature independent. In other cases, it is sufficient to maintain the entire length of the fiber grating at a uniform temperature so that, while the reflected waveband may be shifted, it will not also be distorted. Co-pending Appl. Ser. No. 09/524,862 entitled xe2x80x9cThermally Managed Package for Fiber Optic Bragg Gratingsxe2x80x9d and filed on even date herewith, discloses a package for an optical fiber Bragg grating that includes a retaining element about which the optical fiber is wound. A housing is provided which is adapted to receive the retaining element therein. The housing has a relatively low thermal conductivity and the retaining element has a relatively high thermal conductivity. Such an arrangement ensures that the temperature of the Bragg grating remains substantially uniform even when the ambient temperature undergoes substantial fluctuations. For example, if the exterior of the housing is heated nonuniformly, the low conductivity material from which the housing is formed will appreciably reduce the rate of heat flow into the interior of the housing. Moreover, the heat that does penetrate the housing will be rapidly spread over the entire length of the fiber Bragg grating by the high conductivity member. As a result, the grating will quickly reach a new equilibrium temperature that is uniform along its entirety. Unfortunately, the grating package disclosed in this reference does not also prevent the fiber grating from experiencing substantial strain as the ambient temperature fluctuates.
Accordingly, it would be desirable to provide a package for a fiber Bragg grating such as shown in the previously mentioned patent application, which reduces the strain experienced by the grating as the ambient temperature fluctuates.
In one aspect, the present invention relates to a method for securing an optical fiber Bragg grating to a retaining element equipped with a helical groove. In accordance with the method, an optical fiber Bragg grating is wrapped around the retaining element so that the optical fiber Bragg grating extends in and along the helical groove. Next, an excess length of the optical fiber Bragg grating is provided in the helical groove to substantially alleviate tension exerted upon the optical fiber Bragg grating. Finally, the first and second ends of the fiber Bragg grating are affixed to the retaining element.
In some embodiments of the present invention, the step of providing the excess length of fiber includes the step of wrapping the Bragg grating around the retaining element so that substantially no tension is exerted upon the Bragg grating until a maximum temperature is exceeded. In some cases, the maximum temperature corresponds to a maximum operating temperature of optical fiber Bragg grating.
In other embodiments of the present invention, the step of providing the excess length of fiber includes the steps of securing a spacing element across a plurality of lays of the helical groove, and the wrapping step includes the step of wrapping the optical fiber Bragg grating around both the retaining element and the spacing element so that the optical fiber Bragg grating extends in and along the helical groove.
In still other embodiments of the present invention, the spacing element may be removed after affixing the first and second ends of the fiber Bragg grating to the retaining element.
In other embodiments of the present invention, the spacing element is generally cylindrical in shape. Moreover, the helical groove may be located in an outer surface of the retaining element and the cylindrical spacing element may have a portion of its circumferential surface that conforms to the outer surface of the retaining element.
In another aspect, the present invention relates to a package for an optical fiber Bragg grating. The package includes a retaining element that supports the optical fiber Bragg grating. The retaining element has a helical groove in which the optical fiber extends. The optical fiber is arranged in the groove so that tension exerted upon the optical fiber Bragg grating is substantially alleviated. A housing is also provided which is adapted to receive the retaining element therein.